Charge pump circuit

ABSTRACT

A charge pump circuit and a method of compensating current mismatch in a charge pump circuit. The charge pump circuit comprises a core charge pump circuit; a replica charge pump circuit for sensing a current mismatch in the core charge pump circuit and for converting the sensed current mismatch into a voltage signal V_ctrl; wherein V-ctrl is utilized for compensating the current mismatch in the core charge pump circuit.

FIELD OF INVENTION

The present invention relates broadly to a charge pump circuit and to amethod for compensating current mismatch in a charge pump circuit.

BACKGROUND

A charge pump is an electronic circuit that uses switches to control theconnections of voltages to double voltages, invert voltages, or generatearbitrary voltages, depending on the controller and circuit topology. Anexample for a charge pump circuit application is in phase locked loopcircuits (PLL). For designing a charge pump circuit, an importantobjective is to minimize the mismatch between “up” (pull-up) and “down”(pull-down) currents. In an integer-N synthesizer, the current mismatchwill cause the charge pump's output spectrum to have a higher referencespur. For fractional-N synthesizer, the current mismatch will cause anextra problem, known as higher in-band phase noise.

FIG. 1 shows an existing charge pump circuit that attempts to deal withthe mismatch problem between currents “up” and “down”. In this circuit,the original current source I_bias is mirrored to a common currentbranch for presenting “up” (I_up) and “down” (I_down) currents to N-MOStransistor M1 and P-MOS transistor M2 respectively. Between these twocurrent carrying transistors (M1, M2), there are four trans-gateswitches (S1, S2, S3 and S4) of the same size and they form the currentbranch 101 (S1 and S2) parallel to a dummy branch 100 (S3 and S4). Eachbranch has its two trans-gate switches serially connected. The chargepump output voltage V_ds is taken at the CP_out point between S1 and S2and a reference voltage V_ref is taken between S3 and S4. Linking chargepump output (V_ds) and the voltage reference V_ref, a negative feedbackis formed via an Operational Amplifier (Op) so that the voltage valueV_ref follows V_ds. In these branches, D and U are digital signals froma phase frequency detector (PFD) to control the trans-gate switches (S1to S4) so that the pumping of the positive and negative current (CP_out)is regulated. In this circuit, a charge injection is minimized byimplementing the identical switches (S1 to S4) with a minimal size andthe possible overlap charge injection is reduced by fine-tuning the sizeof current carrying transistors (M1, M2). During operation, M1 and M2are not switched on or off to prevent current switching effects on thedrain of the current sources. When the charge pump is off i.e. both S1and S2 are closed, the current is diverted into a dummy current branch100 via S3 and S4.

In the charge pump circuit, there exists a systematic current variationdue transistor mismatch between M1 and M2. Consequently, the resultingcurrent mismatch of the charge pump circuit is in practice difficult toavoid.

Referring to FIG. 2, the simulation result for the charge pump circuitof FIG. 1 is shown. The vertical axis represents the electric currentvalue and the horizontal axis gives the reference voltage V_ref (0˜1.8V)value, which follows the charge pump output voltage V_ds. The currentpassing through M1 (100 μA) is marked with I_up (curve 200) and thecurrent passing through M2 (−95 μA) is marked with I_down (curve 202).The current mismatch is illustrated by the current_mismatch curve 204.It can be observed that the circuit is not able to compensate thecurrent mismatch and resulted current mismatch is quite large.

Another existing charge pump circuit is illustrated in FIG. 3. Thecharge pump connects an original current source (I_bias) with a feedbacknetwork portion 300, a core charge pump portion 302 and a replica biasportion 304. This circuit uses the replica bias circuit 304 to equalizeup and down currents regardless of the charge pump's output voltageV_ds. However, the voltage range V_ds of this charge pump is narrowwhich inhibits the feedback loop from operating properly. Such chargepump circuits cannot have good current match and are limited in terms ofdynamic voltage range.

There are some charge pump circuits using digital circuits to controlcurrent mismatch. However, the digital circuit has to be turned on atall times to achieve good current match, which causes problems to thecharge pump circuit.

A need therefore exists for compensating current mismatch in a chargepump circuit that seeks to address at least one of the above problems.

SUMMARY

In accordance with a first aspect for the present invention there isprovided a charge pump circuit comprising a core charge pump circuit; areplica charge pump circuit for sensing a current mismatch in the corecharge pump circuit and for converting the sensed current mismatch intoa voltage signal V_ctrl; wherein V-ctrl is utilised for compensating thecurrent mismatch in the core charge pump circuit.

The core charge pump circuit may include a first n-type transistor and afirst p-type transistor, parallel first and second branches betweenrespective drains of the first n-type and the first p-type transistor,each branch including two switch elements, and a voltage followercircuit connected between a V_ref input point and a CP_out point betweenthe switch elements on the first and second branches respectively;

the replica charge pump circuit may include a second n-type transistorand a second n-type transistor, two switch elements of the same type asthe switch elements of the core charge pump circuit connected in seriesbetween the drains of the second n-type and the second p-typetransistor; and a feedback loop with one input taken from a pointbetween the two switch elements of the replica charge pump circuit andV_ref supplied to another input of the feedback loop and V-ctrl as theoutput of the feedback loop.

The charge pump circuit may further comprise a first currentcompensating circuit for converting V_ctrl into a compensating “up”current, and a second current compensating circuit for converting V_ctrlinto a compensating “down” current.

The compensating “up” current may be supplied to the drains of the firstand second n-type transistors, and the compensating “down” current issupplied to the drains of the first and second p-type transistors.

The first and second compensating circuits may comprise respectivedifferential circuits for converting a voltage difference between V_refand V_ctrl into the compensating “up” and “down” currents respectively.

The switch elements may comprise trans-gate switches.

Current mismatch may be substantially compensated over a range of morethan about 1 V in variation of V_ref.

In accordance with a second aspect of the present invention there isprovided a method of compensating current mismatch in a charge pumpcircuit, the method comprising sensing the current mismatch in thecharge pump circuit; converting the sensed current mismatch into avoltage signal V_ctrl; and utilising V-ctrl to compensate the currentmismatch.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readilyapparent to one of ordinary skill in the art from the following writtendescription, by way of example only, and in conjunction with thedrawings, in which:

FIG. 1 shows a circuit diagram of an existing charge pump circuit forcompensating up and down current mismatch;

FIG. 2 shows a simulation result for the current mismatch of the chargepump circuit of FIG. 1;

FIG. 3 shows another existing charge pump circuit for reducing currentmismatch;

FIG. 4 shows a circuit diagram of a charge pump circuit with a replicacharge pump; and

FIG. 5 shows up and down current supply circuits connected to the chargepump circuit of FIG. 4.

FIG. 6 shows simulation results of the disclosed charge pump circuit.

DETAILED DESCRIPTION

Referring to FIG. 4, a charge pump circuit for reducing current mismatchis disclosed. The charge pump comprises three portions, namely a currentsource portion 400, a core charge pump portion 402 and the replicacharge pump portion 404.

The current source portion 400 comprises an original current sourceI_bias and three transistors (Q1, Q2, Q3). These transistors mirror thecurrent from the original current source I_bias to give positive andnegative output currents. In particular, the original current I_biasconnects to the source of Q3 and drain of Q1. The drain of Q1 and gatesof Q1 and Q2 are joined together. The sources of Q1 and Q2 are connectedto ground. The gate of Q3 is connected to its drain and further to thedrain of Q2. The source of Q3 as well as the original current sourceI_bias are connected to a positive supply voltage V_dd.

The core charge pump circuit 402 is connected to the mirrored currentsources for receiving up and down currents. At the up current side ofM1, the source of M1 is connected to the source of Q3 and the gates ofM1 and Q3 are jointly connected to the drain of Q2. At the down currentside, the source of M2 connects the sources of Q1 and Q2 to ground. Thegates of M2, Q2 and Q1 are connected together to the original currentsource I_bias directly.

Between the up and down current carrying transistors M1 and M2, thereare two parallel branches comprising four trans-gate switches, namelyS1, S2, S3 and S4. S1 and S2 are serially connected between M1 and M2 toform a current branch 406 and the charge pump output V_ds (CP_out) istaken between the trans-gate switches S1 and S2. Parallel to S1 and S2,S3 and S4 are serially connected to form a dummy branch 408 between M1and M2 where the voltage reference V_ref is taken between the switchesS3 and S4. The reference voltage V_ref is also at the output of anOperational Amplifier (OPAMP) Op 1 and control signals D and U for thetrans-gate switches (S1, S2, S3 and S4) are taken from a Phase FrequencyDetector (PFD).

Between the current branch 406 and the dummy branch 408, a negativefeedback loop 410 (voltage follower) is formed by Opt between the chargepump output 411 and the reference voltage V_ref.

The replica charge pump circuit 404 is used for sensing the currentmismatch of the core charge pump 402. Within the replica charge pump inout 404, a feedback loop 412 is used to convert the sensed currentmismatch into a voltage signal, V_ctrl.

The replica charge pump circuit 404 has two transistors (M3 and M4) andtwo trans-gate switches (S5 and S6). M3 and M4 have the same size as M1and M2 while S5 and S6 have the same size as S1 to S4. M3, S5, S6 and M4are connected in series. The current mismatch due to the size mismatchand the drain voltages of transistors M1 and M2 can be compensated inthe implementation shown in FIG. 4. It will be appreciated by a personskilled in the art that for M1 and M3, and for M2 and M4, a good matchcan be achieved, typically less than 1% in real circuits. The trans-gateswitches S5 and S6 are maintained at an on state continuously during thecharge pump's circuit 401 operation. Between the trans-gate switches S5and S6, a connection marked as point A is coupled to a secondOperational Amplifier Op2 to form the negative feedback loop 412.

The feedback loop 412 of the replica charge pump circuit 404 is formedaround the Operational Amplifier Op2. Here, Op2 is a rail-to-rail OPAMPand it is used as trans-impedance amplifier (TIA). In particular, theinput from point A is linked to the inverting input 414 and a controlvoltage V_ctrl is fed back from the output 416 of Op2 to the invertinginput 414 via a resistor R. On the other hand, the reference voltageV_ref is supplied to the non-inverting input 418 of Op2.

For the “external” connections of the replica charge pump circuit 404,the source and gate of M3 are connected to the source and gate of M1respectively. Similarly, the source and gate of M4 are connected to thesource and gate of M2 respectively.

During operation, the current mismatch of the original charge pumpoutput V_ds (at CP_out) is mirrored to point A by the replica chargepump circuit 404 and supplied to the second Operational Amplifier Op2,which is used as TIA. If the “up” current on M1 is lower than the “down”current, V_ctrl becomes higher than V_ref. If the “up” current is higherthan the “down” current, V_ctrl becomes lower than V_ref. Thus, themismatch in the “up” and “down” current is effectively converted into adifferential voltage signal V_ctrl. Additional differential circuits areused to convert the V_ctrl into compensating current signals. Thedifferential circuits are described below with reference to FIGS. 5( a)and (b).

The disclosed charge pump circuit of FIG. 4 has four additional currentinput connecting, labeled as I_1, I_2, I_3 and I_4. I_1 and I_3 are forreceiving compensation “up” current while I_2 and I_4 are for receivingcompensation “down” current. The compensating “up” current supplycircuit is shown in FIG. 5( a) and the compensating “down” currentsupply is illustrated by FIG. 5( b).

Referring to FIG. 5( a), the compensating “up” current source 500 isregulated by V_ref and V_ctrl. In particular, there are seventransistors (Q4 to Q7 and M5 to M7) forming the current source. Q4 to Q7form source loop 501 and M5 to M7 are connected in parallel. Within theloop, Q6's drain connects to Q4's drain and Q7's drain connects to Q5'sdrain. The sources of Q4 and Q5 are connected together to ground via acurrent source 502. The sources of Q6 and Q7 are connected to a positivevoltage V_dd. The gates of Q6 and Q7 are also connected to the drain ofQ4. V_ref is applied to the gate of Q4 and V_ctrl is applied to the gateof Q5. For M5 to M7, all sources are connected to the sources of Q6 andQ7 and all gates are connected to the drain of Q5. The gate of M5 isalso connected to the drain of Q5. The “up” current output to node I_1and I_3 are taken from the drains of M6 and M7 respectively.

Referring to FIG. 5( b), the compensating “down” current is alsoregulated by V_ref and V_ctrl. In particular, there are seventransistors (Q8 to Q11 and M8 to M10) forming the current source 511. Q8to Q11 form a loop source 510 and M8 to M10 are connected in parallel.Within the loop 510, Q10's drain connects to Q8's drain and Q11's drainconnects to Q9's drain. The sources of Q8 and Q9 are connected toground. The sources of Q10 and Q11 are connected to a positive voltageV_dd via a current source 512. The gates of Q8 and Q9 are connected tothe drain of Q10. V_ref is applied to the gate of Q10 and V_ctrl isapplied to the gate of Q11. For M8 to M10, the sources are connected tothe sources of Q8 and Q9 at ground and all gates are connected to thedrain of Q9. The gate of M8 is also connected to the drain of Q9. Thecompensating “down” current output I_2 and I_4 are taken from the drainsof M8 and M9 respectively. It will be appreciated that sources 502 and512 may be mirrored from one current source.

The above-disclosed charge pump circuits provide current mismatchfeedback. The feedback will compensate the current mismatch and force“up” and “down” currents closer over a wider V_ds range.

A simulation result of the disclosed charge pump circuit is presented inFIG. 6. The vertical axis and horizontal axis denotes the mismatchcurrent value in μA and reference voltage value V_ref respectively. Inthe graph, the “up” current I_up (curve 600) and down current I_down(curve 602) are plotted together with the curve 604 of current mismatch.The graph shows that the current mismatch is less than about 1% when thereference voltage V_ref varies from about 0.2V to about 1.5V, i.e.providing a V_ref range of more than about 1V. Also shown in FIG. 6 arethe compensating “up” and “down” currents in curves 606 and 608respectively.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

1. A charge pump circuit comprising: a core charge pump circuit; areplica charge pump circuit for sensing a current mismatch in the corecharge pump circuit and for converting the sensed current mismatch intoa voltage signal V_ctrl; wherein V-ctrl is utilized for compensating thecurrent mismatch in the core charge pump circuit; the core charge pumpcircuit includes a first n-type transistor and a first p-typetransistor, parallel first and second branches between respective drainsof the first n-type and the first p-type transistor, each branchincluding two switch elements, and a voltage follower circuit connectedbetween a V ref input point and CP out point between the switch elementson the first and second branches respectively; the replica charge pumpcircuit includes a second n-type transistor and a second n-typetransistor, two switch elements of the same type as the switch elementsof the core charge pump circuit connected in series between the drainsof the second n-type and the second p-type transistor; and a feedbackloop with one input taken from a point between the two switch elementsof the replica charge pump circuit and V ref supplied to another inputof the feedback loop and V-ctrl as the output of the feedback loop; andfurther comprising a first current compensating circuit for converting Vctrl into a compensating “up” current, and a second current compensatingcircuit for converting V ctrl into a compensating “down” current.
 2. Thecharge pump circuit as claimed in claim 1, wherein the compensating “up”current is supplied to the drains of the first and second n-typetransistors, and the compensating “down” current is supplied to thedrains of the first and second p-type transistors.
 3. The charge pumpcircuit as claimed in claim 1, wherein the first and second compensatingcircuits comprise respective differential circuits for converting avoltage difference between V_ref and V_ctrl into the compensating “up”and “down” currents respectively.
 4. The charge pump circuit as claimedin claim 1, wherein the switch elements comprise transgate switches. 5.The charge pump circuit as claimed in claim 1, wherein current mismatchis substantially compensated over a range of more than about 1V invariation of V_ref.
 6. A method of compensating current mismatch in acharge pump circuit, the method comprising: sensing the current mismatchin the charge pump circuit; converting the sensed current mismatch intoa voltage signal V_ctrl; and utilizing V-ctrl to compensate the currentmismatch; wherein a core charge pump circuit includes a first n-typetransistor and a first p-type transistor, parallel first and secondbranches between respective drains of the first n-type and the firstp-type transistor, each branch including two switch elements, and avoltage follower circuit connected between a V ref input point and a CPout point between the switch elements on the first and second branchesrespectively; a replica charge pump circuit includes a second n-typetransistor and a second n-type transistor, two switch elements of thesame type as the switch elements of the core charge pump circuitconnected in series between the drains of the second n-type and thesecond p-type transistor; and a feedback loop with one input taken froma point between the two switch elements of the replica charge pumpcircuit and V ref supplied to another input of the feedback loop andV-ctrl as the output of the feedback loop; and using a first currentcompensating circuit for converting V ctrl into a compensating “up”current, and using a second current compensating circuit for convertingV ctrl into a compensating “down” current.